1. Field of the Invention
The present invention relates to an exposure control device for use in a camera and, more particularly, to an exposure control device that reliably controls exposure even under cloudy weather conditions.
2. Description of the Related Art
In a known technique to achieve an optimum exposure in a camera, an exposure control device photometers a photographic object with a photometric sensor having a plurality of photosensitive elements and determines whether the photographic object is positioned in a back lighting condition based on a plurality of luminance signals output by the photometric sensor. U.S. Pat. No. 5,596,387 discloses a technique in which improved exposure control is performed by obtaining both luminance information and color temperature information based on the spectral component of the object field, by imparting different spectral characteristics to a plurality of photosensitive elements using a saturated color filter or a complementary color filter. According to U.S. Pat. No. 5,596,387, a photograph which conforms to the image of the time band of photographing in the photographing of an evening scene or a morning scene is produced, by performing exposure control based on the luminance information and the color temperature information of the object field.
To take a nice picture by providing an appropriate exposure in outdoor photographing, weather conditions need to be considered. As already discussed, a technique is known which achieves an appropriate exposure by determining front lighting or back lighting during good weather conditions. In contrast, no technique has been available to provide optimum exposure during cloudy weather conditions.
Specifically, a difference between good weather conditions and cloudy weather conditions is discussed. During good weather conditions under which a luminance of Bv≈9 results with a 18% gray reflective plate photometered in the back lighting, a luminance Bv≈8 to 9 results when the luminance of a blue sky portion of the sky is measured at a high elevation angle. A luminance of Bv≈9 to 10 results when the luminance of a white portion of the sky is measured at a low elevation angle. It is said that the luminance of an object is appropriately obtained if the 18% gray reflective plate is photometered. Even if sky appears in the object field, the difference between the luminance of the sky and the luminance of the 18% gray reflective plate is about one notch in the Bv value, and the exposure of the object is not largely in error as long as the photographing is not performed in the back lighting.
When the luminance of a uniformly light gray sky portion is photometered under a cloudy weather condition, a luminance of Bv≈10 results. In contrast, a luminance Bv≈6 to 7 results when the 18% gray reflective plate is photometered. In this case, the luminance difference between the sky portion and an ordinary object portion is three in the Bv value. When the sky portion appears in the object field during cloudy weather, the output of the photometric sensor of a camera is strongly affected by a high luminance of the sky portion. As a result, the object is subject to an under-exposure under the cloudy weather condition in contrast to good weather conditions. There is room for improvements in exposure control.
In accordance with the method of the present invention, an area having high lightness and low saturation is determined, and the area is then determined to be a cloudy sky area when an object range in the area is distant and when the area is on the side of the sky in the object field. Exposure control is performed in such a manner that the luminance of the area determined to be a cloudy sky area is weighted for a smaller value. In this way, under-exposure is avoided when photographing under a cloudy weather condition featuring high lightness and low saturation.
An area is determined to be a cloudy sky area on condition that an area results in a large visible-light luminance, that the object range in the area is distant, and that the area is present on the side of the sky in the object field. Since the luminance of the determined cloudy sky area is weighted for a smaller value in exposure control, under-exposure is prevented when photographing under a cloudy weather featuring a low saturation and a high luminance.
The present invention in one aspect relates to an exposure control device for use in a camera, and includes a photosensor having at least two different types of spectral characteristics, a lightness calculating circuit that calculates a lightness of each zone of the photosensor from the output of the zone of the photosensor, the photosensor being partitioned into a plurality of zones, a saturation calculating circuit that calculates saturation of each zone of the photosensor from the output of the zone of the photosensor, a cloudy sky determining circuit that determines whether an area of the object field is a cloudy sky area, based on the lightness and the saturation of each zone of the photosensor, and an exposure control circuit that performs exposure control based on the determination result of the cloudy sky determining circuit.
The present invention in another aspect relates to an exposure control device for use in a camera, and includes a photosensor having at least two different types of spectral characteristics, a range finder unit that measures a range to an object, a posture detector unit that detects a posture of the body of the camera, a lightness calculating circuit that calculates lightness of each zone of the photosensor from the output of the zone of the photosensor, the photosensor being partitioned into a plurality of zones, a saturation calculating circuit that calculates saturation of each zone of the photosensor from the output of the zone of the photosensor, a particular area determining circuit that determines whether an area of the object field is a particular area, based on the lightness and the saturation of each zone, a cloudy sky determining circuit that determines whether the particular area is a cloudy sky area, based on the range to the object and the posture of the camera, and an exposure control circuit that performs exposure control based on the determination result of the cloudy sky determining circuit.
The present invention in yet another aspect relates to an exposure control device for use in a camera, and includes a photosensor having at least two different types of spectral characteristics, a range finder unit that measures a range to an object, a posture detector unit that detects a posture of the body of the camera, a lightness calculating circuit that calculates lightness of each zone of the photosensor from the output of the zone of the photosensor, the photosensor being partitioned into a plurality of zones, a saturation calculating circuit that calculates saturation of each zone of the photosensor from the output of the zone of the photosensor, a particular area determining circuit that determines whether an area of the object field is a particular area having a lightness thereof higher than a lightness reference value and a saturation thereof lower than a saturation reference value, a cloudy sky determining circuit that determines that the particular area is a cloudy sky area when the range of an object in the particular area is distant, and when the particular area is present on the side of the sky in the object field, based on the outputs of the range finder unit and the posture detector unit, an optimum luminance calculating circuit that calculates an optimum luminance for exposure in accordance with the area size of the cloudy sky area in the object field, and an exposure control circuit that performs exposure control using the calculated optimum luminance.
The present invention in still another aspect relates to an exposure control device for use in a camera, and includes a photosensor having spectral characteristics in the visible-light region and in the infrared-light region, a visible-light luminance calculating circuit that calculates visible-light luminance in each zone of the photosensor from the output of the zone of the photosensor, the photosensor being partitioned into a plurality of zones, an infrared-light luminance calculating circuit that calculates infrared-light luminance in each zone of the photosensor from the output of the zone of the photosensor, a cloudy sky determining circuit that determines whether an area of the object field is a cloudy sky area, based on the visible-light luminance and the infrared-light luminance in each zone, and an exposure control circuit that performs exposure control based on the determination result of the cloudy sky determining circuit.
The present invention in still another aspect relates to an exposure control device for use in a camera, and includes a photosensor having spectral characteristics in the visible-light region and in the infrared-light region, a range finder unit that measures a range to an object, a posture detector unit that detects a posture of the body of the camera, a visible-light luminance calculating circuit that calculates visible-light luminance in each zone of the photosensor from the output of the zone of the photosensor, the photosensor being partitioned into a plurality of zones, an infrared-light luminance calculating circuit that calculates infrared-light luminance in each zone of the photosensor from the output of the zone of the photosensor, a sky area determining circuit that determines whether an area of the object field is a sky area, based on the visible-light luminance and the infrared-light luminance in the zone and the posture of the body of the camera, a cloudy sky determining circuit that determines whether a sky area is a cloudy sky area, based on a value of the ratio of the visible-light luminance to the infrared-light luminance, and an exposure control circuit that performs exposure control based on a determination result of the cloudy sky determining circuit.
The present invention in yet another aspect relates to an exposure control device for use in a camera, and includes a photosensor having spectral characteristics in the visible-light region and in the infrared-light region, a range finder unit that measures a range to an object, a posture detector unit that detects a posture of the body of the camera, a visible-light luminance calculating circuit that calculates visible-light luminance in each zone of the photosensor from the output of the zone of the photosensor, the photosensor being partitioned into a plurality of zones, an infrared-light luminance calculating circuit that calculates infrared-light luminance in each zone of the photosensor from the output of the zone of the photosensor, a sky area determining circuit that determines whether an area is a sky area where the visible-light luminance is higher than a reference value, the range of the object is distant, and the area is on the sky side in the range of the object field, a cloudy sky determining circuit that determines whether a sky area is a cloudy sky area where the infrared-light luminance is higher than the visible-light luminance in ratio, an optimum luminance calculating circuit that calculates an optimum luminance for exposure in accordance with an area size of the cloudy sky area in the object field, and an exposure control circuit that performs exposure control using the calculated optimum luminance.
Further objects, features, and advantages of the present invention will be apparent from the following description of the preferred embodiments with reference to the attached drawings.